Gas and Dust in the Inner Few Degrees of the Galaxy

Abstract

This review discusses the complex morphology and perturbed kinematics, as well as the outstanding physical characteristics of the interstellar medium within a few 100 pc of the galactic center. A total of ~10 7.9 Mo of dense molecular gas, representing ~10% of the Galaxy's neutral mass content, has settled in a thin layer [size: 450 × 40 pc], composed of giant molecular cloud complexes. The spatial distribution is highly asymmetric with respect to the center, and motions differ considerably from equilibrium conditions. The dynamical situation is still obscure (explosive event vs. response to distorted gravitational potential), but any disturbance must have occurred quite recently (τ dy ~10 6 yr). Evidence for large–scale star–forming activity is reviewed, and for a standard IMF, a total star formation rate Φ ~0.5 M 0 yr −1 is inferred. The galactic center clouds differ considerably in their physical and chemical characteristics from GMC's in the outer Galaxy. Pervasive high bulk gas temperatures (T kin ~70 K), mean H 2 densities of n ~10 4 cm −3 , and linewidths ≥10–20 kms −1 are probably a consequence of the clouds' location in the steep gravitational potential of the central bulge.